Slipping movements can easily occur in portable motor-driven chain saws, if the part of the chain saw extending over the top side of the blade plate comes into contact with a solid object such as, for instance, a branch. If the saw is thrown upwards as a result it can injure the operator in the face or the upper part of the body if he or she releases the front, bowed handle of the saw. To prevent this, protective devices are already known which brake the chain if the saw suddenly makes a slipping movement. It is known to use latch-like triggering devices to trigger brake devices for braking the saw chain of a portable motor-driven chain saw.
For instance, a brake device for motor driven saws is known which has single-arm latch levers pivotably mounted on one side and connected to a tension spring and the brake band engaging around the drum of the driving chain wheel. In this brake device there is the disadvantage that relatively considerable forces must be exerted on the latch lever to unlatch the locking latch. This causes the risk of delayed response by the brake device, so that the operator is not adequately protected (German OS Nos. 22 17 707 and 24 40 483).
A brake device is also known in which a triggering bowed member is provided on which a cam is formed which can be brought into engagement with the stop of a sprung actuating member connected to the free end portion of the brake band. However, in this system vibrations and impacts, such as occur during normal sawing operation, are transmitted to the triggering bowed member and therefore easily release the connection to the actuating member. Precisely when sawing knotty timber or the like, the brake system an easily be triggered without a dangerous situation having arisen, so that a brake system constructed in that way contributes towards reducing the operator's performance (U.S. Pat. No. 3,982,616, German OS No. 24 59 528).
A brake system is also known which is expensive to construct, because it uses a two-armed lever. Moreover, pivoting movements of the saw blade, which occur even during normal sawing operation, are transmitted directly via the lever to the brake device, so that in this case also the brake device can respond without the operator being in danger (German OS No. 26 21 812).
In the construction disclosed in U.S. Pat. No. 3,934,345 the ends of the brake band are attached directly to the triggering lever.
PCT Patent Specification No. WO 80/00548 provides a type of articulation of the ends of the brake band by which the brake band is attached by one end to the triggering lever, its other end being attached to the machine casing.
The brake band is guided in a very complicated manner in German OS No. 28 31 097. In that prior art construction the brake band is attached at one end to the machine casing, the other end of the brake band being attached to the triggering lever after numerous deflections over pins and bolts.
In the brake device disclosed in German OS No. 26 02 247 one end of the brake band is attached to the machine casing, while the other end of the brake band is articulated to the free end of a sprung toggle lever actuable by the triggering lever via an adjusting member.
It is also known for the brake band to be attached at one end to the triggering lever, while the other end of the brake band is guided over a pin and then attached to the machine casing (German OS No. 27 16 448, U.S. Pat. No. 4,059,895),
A chain saw brake device operating with a toggle joint is known from U.S. Pat. No. 4,077,125. In that brake device the triggering lever is connected via a sprung control rod to the two levers of the toggle joint, the free end of the actuating rod engaging with the joint connecting joint of the two levers. The band brake is tightened by moving the two levers of the toggle joint into an angular position on actuation of the triggering lever, although the band brake is not attached by its two ends directly to one of the two levers of the toggle joint.
All the known brake devices therefore share the feature that the brake band is attached by one end to the casing receiving the brake device as by the other end to the lever of an actuating device, and even in that embodiment is which a toggle joint is provided for actuating the band brake, the band brake ends are not directly connected to the toggle joint. There is also the fact that some of the known constructions of brake devices are of complicated construction.
In addition, motor-driven chain saw safety devices are known which have a mass member which responds to an acceleration force, and are operatively connected to a brake device. In such a safety device the mass member which triggers the brake device can be displaced from its inoperative position to deliver a pulse by acceleration forces operating in a single plane. The safety device comes into operation if uncontrolled acceleration forces occur, for instance, with upward jerks of the saw blade.
A motor-driven chain saw is also known which has a safety device having a mass member responding to acceleration force and operatively connected to a brake device, the mass member having at least one bearing part under a retaining force and being disposed as a monostable inertia switch displaceable thereby under the influence of the accelerating force. This safety device ensures that the mass member triggers, circularly on all sides, a contact for switching on the brake device (German OS No. 30 18 952).